The cetane number of a fuel composition is a measure of its ease of ignition and combustion. With a lower cetane number fuel a compression ignition (diesel) engine tends to be more difficult to start and may run more noisily when cold; conversely a fuel of higher cetane number tends to impart easier cold starting, to alleviate white smoke (“cold smoke”) caused by incomplete combustion after starting and to have a positive impact on emissions such as NOx and particulate matter during engine operation.